Why plants, fungi, bacteria and animals belonging to different kingdoms of nature are the objects

Why plants, fungi, bacteria and animals belonging to different kingdoms of nature are the objects of study of one science-biology.

Plants are studied separately, because they are distinguished by a certain morphology and anatomy, they receive energy through photosynthesis, they are immobile and are strikingly different from animals and bacteria.
Mushrooms could be considered together with plants, but their nutrition is saprotrophic and osmotrophic. They absorb nutrients on the surface of the body. The differences are primarily at the level of the cell, many polymers, and the way of life.
Bacteria according to the cellular structure of a prokaryote and already on this basis should be considered in a separate science. Of no less interest is their method of transferring genetic information among themselves – horizontal gene transfer.
Animals are eukaryotes, they are mobile, feed through special holes, for the most part they have a nervous system and a specific metabolism.
Plants are studied by botany, fungi by mycology, bacteria by microbiology, animals by zoology.